The present invention relates to optical members such as lenses, prisms, filters, windows, reflectors, and etalon plates used in various apparatus employed in environments where they are exposed to high-power ultraviolet light having a wavelength of about 360 nm or less, e.g. an excimer laser beam, a YAG fourth harmonics (250 nm) laser beam, or some other high-power ultraviolet laser beam. Further the invention relates to semi-finished products which have not yet been finally polished, and to blanks, and more specifically to lenses, prisms, etc. assembled into a device such as a lithographic laser exposure apparatus for producing highly integrated circuits, a laser fabrication apparatus, a medical apparatus, a nuclear fusion apparatus, or some other apparatus which uses a high-power ultraviolet laser beam. Still further, the invention relates to a method of producing such optical members and/or blanks for such optical members.
In recent years there has been remarkable progress in making large scale integrated circuits (LSIs) finer and in increasing the degree of integration of LSIs. VLSIs having 1,000,000 or more elements per chip are now starting to become prevalent. Along with this progress, lithography techniques for drawing an integrated circuit pattern on a wafer have developed rapidly, and techniques capable of drawing a line with an even narrower width are now being developed. For example, a pattern line having a width of 1 .mu.m which is suitable for 1M bit DRAM, and a pattern line having a width of 0.8 .mu.m which is suitable for 4M bit DRAM, are now being developed. These techniques all involve photolithography.
In the field of lithography, there is an urgent need to develop techniques of drawing at submicron scale, i.e. with pattern line widths from 0.5 .mu.m to 0.2 .mu.m, which are suitable for 16M bit DRAMs to 256M bit DRAMs which are expected to be realized in the near future. In view of the steady progress of modern optical systems, light sources, photoresists, etc., it is anticipated that photolithography will also play a major role in such ultrafine line width drawing techniques. Indeed, photolithography has various attractive features needed for ultrafine line width drawing because there are, for example, light sources having relatively high brightness, highly sensitive resists, and stable optical materials. However, photolithography has the problem that the resolution is limited by diffraction due to the long wavelengths which are used. To solve this problem, the numerical aperture (NA) of optical systems must be enlarged and/or the wavelength of the light must be shortened.
With respect to enlargement of the numerical aperture of optical systems, numerical apertures of not less than 0.4 are now available, and a lens having a numerical aperture of 0.6 has been developed as a trial product. But, as the numerical aperture is increased, the depth of focus decreases. Consequently, there is a limit to how much the numerical aperture can be increased in order to improve the resolution. Therefore, shortening the wavelength of the light is being considered.
If ultraviolet radiation having a wavelength of 400 nm or less is used with a lens of conventional optical glass, however, the light transmittance rapidly decreases at a working wavelength of 365 nm (i-line). In other words, light absorption and generation of heat due to the light absorption occur, and consequently the focal point of the lens and other properties of the lens are disturbed.
To obviate such difficulties, it has been suggested to use silica glass as the lens material instead of conventional optical glasses. But when normal ultraviolet radiation is passed through silica glass, chromatic aberration occurs because the spectrum of the light is so broad. Thus, it also has been proposed to use a laser beam which oscillates in the ultraviolet range and has a narrow spectrum width as a light source for photolithography.
Among the lasers used for photolithography, excimer lasers are the most practical. An excimer laser is a high-power pulse laser oscillating in the ultraviolet range, e.g. at a wavelength ranging from about 360 nm to about 150 nm. Excimer lasers have the most powerful energy density of known ultraviolet light sources. Various working gas mixtures are possible, including XeF (351 and 353 nm), KrF (248 nm), XeCl (308 nm), ArF (193 nm), etc. Of these, it is preferred for reasons of oscillation efficiency and gas life, to use KrF (248 nm) or ArF (193 nm) which have shorter wavelengths in order to obtain a clearer image at a submicron scale.
It has been found that the irradiation of an excimer laser on silica glass adversely affects the optical properties of the glass. That is to say, applicants have discovered that optical members formed of silica glass are highly subject to optical damage when the silica glass is exposed to an ultraviolet laser beam having a wavelength ranging from 360 nm to 150 nm. Since an excimer laser beam is quite high in power compared to a conventional i-line source or g-line source, etc., as the oscillation wavelength is shortened, even if the optical members for the laser beam are made of silica glass, when the optical members are exposed to the laser beam for an extended period of time, problems arise in that the optical members, including the lenses, are damaged, and their optical properties are changed, e.g., the transmittance decreases, etc. More particularly, irradiation with an ultraviolet laser beam for a long time results, for example, in breaking of the network structure of silica glass, producing an absorption peak at about 215 nm (the so-called E' center) and another absorption band at about 260 nm, thereby decreasing the transmittance in the range from 360 nm to 150 nm and causing optical deterioration. Even high-purity synthetic silica glass is subject to optical damage upon exposure to an ultraviolet laser beam. Such deterioration is not serious with conventional i-line (365 nm) or g-line (436 nm) light sources, but becomes serious for shorter wavelength ultraviolet light beams ranging from 360 nm to 150 nm. Eventually cracks appear. In contrast thereto, other types of light which have longer wavelengths, such as visible light, have a negligible influence on the glass.
One cause of the deterioration in the optical properties is attributed to the presence of metal impurities in the silica glass. Therefore, optical members such as lenses, etc. for a laser beam have been formed of synthetic silica glass made using a highly purified silicon compound, such as SiCl.sub.4, as a raw material instead of natural quartz. However, even optical members formed from highly pure silica glass have not given satisfactory results for a high-power laser beam having a short wavelength for several reasons. The first reason is that even if it is intended to make highly pure silica glass, it is impossible to completely eliminate the presence of metal impurities because of problems inherent in the raw material and in the production process of silica glass. A second reason is that synthetic silica glass appears to include various structural defects which decrease the laser beam resistance. These two factors combine to cause the laser resistance to deteriorate. No techniques are known which have been developed to enable an optical member of synthetic silica glass to resist optical deterioration when exposed to short wavelength ultraviolet laser radiation.
It is known from Japanese Patent Publication Sho 40-10228 that colorization of a silica glass article made by melting natural quartz due to the influence of ionizing radiation can be prevented by heating the article in an atmosphere of hydrogen gas to about 400.degree. to 1,000.degree. C., but this publication only teaches occlusion (doping) of hydrogen gas in order to prevent or inhibit solarization to some degree in natural quartz glass. Mere hydrogen doping does not, however, prevent degradation of the optical properties of silica glass optical members subjected to a high-power, ultraviolet light beam like the irradiation of an excimer laser. Thus, this publication does not enable persons skilled in the art to prevent deterioration over time of the optical properties of synthetic glass optical elements exposed to a high-power ultraviolet laser.
It is also known from Japanese Patent Publication Sho 39-23850 that the transmittance of ultraviolet light by a silica glass body can be improved by heat treating the glass body in a hydrogen atmosphere at 950.degree. to 1,400.degree. C. and then heat treating the glass body again in an oxygen-containing atmosphere at 950.degree. to 1,400.degree. C. The purpose of this treatment is to increase the transmittance of ultraviolet light having a wavelength of 300 nm or less by decreasing the metal impurity content which remains in quartz glass and affects the light absorption of the glass. Again this publication provides no information about how to prevent progressive deterioration over time of the optical properties of optical members exposed to ultraviolet radiation. Furthermore, since this prior technique carries out a heat treatment in oxygen after the heat treatment in hydrogen, the initially doped hydrogen is removed by the subsequent oxygen atmosphere heat treatment, thereby eliminating the effect of the hydrogen heat treatment. This document also describes that the heat treatments in the hydrogen atmosphere and in the oxygen atmosphere each give a maximum effect at the temperature of 1,400.degree. C. However, heat treatment at such a high temperature results in contamination of the glass with impurities from the furnace, and thereby produces glass containing significant amounts of impurities which may adversely affect the resistance of the glass to optical deterioration from exposure to high-power, ultraviolet laser light.
U.S. Pat. No. 4,553,995 discloses that hydrogenating an optical fiber or image fiber preform at an elevated temperature also can improve attenuation increases which arise when drawn optical fibers are exposed to gamma-radiation by eliminating bond defects which contribute to bond breakage during the fiber drawing process. Japanese Patent Publication Sho 58-125635 indicates that gamma-radiation resistance of quartz glass optical fibers may be improved by increasing the OH group concentration of the fiber core to greater than 1000 ppm and optionally adding fluorine or chlorine, however no explanation is given of how to accomplish this. In neither of these publications is there any information about resistance to high-power, ultraviolet laser light.
Another problem in the production of optical members for use in very high precision optical devices, such as lithography exposure apparatus for exposing submicron scale integrated circuit patterns, is the attainment of uniform optical properties, particularly a homogeneous refractive index.
In general, optical members of the type to which the present invention applies are manufactured by a process in which the starting glass ingot is remolded into the desired shape, such as a column, disk or sphere, and then annealed (heating followed by slow cooling) to eliminate residual strain. Then the peripheral portion of the blanks is ground off, and finally the blanks are cut, polished and film-coated to fabricate optical members. In this production process, the annealing step is very necessary.
However, even if the annealing speed in the annealing step is slow as possible, it is impossible to get the same cooling rate in both the peripheral zone and the central zone. The cooling rate at the periphery is inevitably faster than at the center. This causes a fictive temperature distribution, the shape of which is a concave curve ascending from the central zone towards the peripheral zone in the cross section of the blanks.
The concept of a fictive temperature distribution is believed to arise from the fact that silica does not undergo an abrupt crystalline transformation with changes in temperature. Instead, the physical properties of the silica change subsequently as a result of a change in temperature, and the amount of change in the physical properties varies depending on the rate of temperature change. If two pieces of silica glass are cooled at different rates, one suddenly and the other gradually, the one which is cooled suddenly will undergo a lesser change in properties than the one which is cooled gradually. In other words, the physical properties such as density, refractive index, etc. of the piece which is cooled suddenly will be more nearly equivalent to the properties at a higher temperature, termed the fictive temperature, even though both pieces are cooled to the same actual temperature. See Brueckner, J. Non-Crystalline Solids, Vol. 5, pp. 123-175 (1970).
If a fictive temperature difference is permitted to remain as optical blanks cool to room temperature, then even if a glass blank with ideal uniformity in chemical composition is annealed, the refractive index distribution in the annealed blank inevitably varies with the fictive temperature distribution, resulting in a refractive index distribution having the shape of a concave curve correlated to the fictive temperature distribution.
Therefore, in order to obtain blanks having a uniform refractive index, it is necessary both to make the silica glass highly pure by synthesis and to make the fictive temperature uniform by the heat treatment. Improvements in the uniformity of the temperature, however, are limited so long as the cooling rate is substantially finite, even if improvements in the heat-treating furnace and the heat treatment temperature program are made.
When conventional light, such as a g-line source (436 nm), is used, the presence of such refractive index distributions does not cause serious problems. However, in optical members used for short wavelength laser beams (193 nm to 308 nm), and particularly in laser exposure apparatus for submicron scale lithography having a pattern line width of, for example, 0.5 .mu.m, the refractive index inhomogeneity (.delta.n) needs to be smaller by at least a factor of 10 or more than in the case of visible light. But in optical members fabricated from silica glass blanks having poor optical homogeneity, it is difficult to obtain a highly homogeneous refractive index, and it is therefore impossible to achieve lithography of a fine, clear, submicron scale image.
Thus, in order to facilitate the use of ultraviolet light sources having narrow wavelengths in the range from 360 mn to 150 nm, such as excimer lasers, in high precision optical devices such as exposure apparatus for submicron integrated circuit pattern lithography, there is a need for ways to provide optical members with uniform optical properties including a highly homogeneous refractive index and to prevent the optical properties of such optical members from deteriorating under exposure to light from a high-power ultraviolet light beam like that of an excimer laser, and for optical members having uniform optical properties including a highly homogeneous refractive index whose optical properties do not deteriorate over time when exposed to a high-power, ultraviolet laser beam such as an excimer laser.